Certain azolyloxy-carboxylic acid amides and heteroaryloxy-acetamides of the general formula R--O--CH(R.sup.1)--CO--N(R.sup.2)(R.sup.3) are known to have herbicidal activity (See, e.g., U.S. Pat. Nos. 4,756,741 and 5,101,034). U.S. Pat. No. 5,101,034 discloses a particular class of heteroaryloxyacetamides, namely thiadiazole acetamides as having herbicidal activity. The thiadiazole acetamides are made by reacting a thiadiazole sulfone with an hydroxyacetanilide in acetone. Of particular relevance to the present invention is the disclosure of a synthetic scheme for making 2-(5-trifluoromethyl)-1,3,4-thiadiazol-2-yl-oxy)-N-methylacetanilide. In accordance with that synthetic scheme, 2-methylsulfonyl-5-trifluoromethyl-1,3,4-thiadiazole is reacted with N-methyl-2-hydroxyacetanilide, potassium carbonate, and tetraethylammonium bromide. Acetone is used as the solvent for the reaction. The reaction is carried out at a temperature of 20.degree. C.-25.degree. C. for 20 hours. Undissolved salts are filtered off and washed with acetone. The filtrate is freed of solvent in vacuo and the resulting residue taken up in diethyl ether, washed with dilute hydrochloric acid, dried and filtered. After freeing the filtrate of solvent, the end product is crystallized from the oily residue. Reported yields are about 90%.
U.S. Pat. Nos. 4,756,741 and 4,645,525 disclose a synthetic scheme for making O-(2-trifluoromethyl-1,3,4,-thiadiazol-5-yl-oxo)acetic N-methylanilide. In accordance with that scheme, 2-hydroxy-acetic acid-N-methylanilide is reacted with dimethylsulfoxide and calcium oxide at 50.degree. C. for 1 hour. 5-Bromo-2-trifluoromethyl-1,3,4-thidiazole is then added to the reaction mixture and the mixture stirred at 50.degree. C. for 40 hours. The mixture is then poured into water and the oil that precipitates is extracted with methylene chloride. The end product is obtained in about 90% yield by distilling off the methylene chloride.
U.S. Pat. No. 4,585,471 discloses synthetic schemes for making (5-trifluoromethyl-1,3,4-thiadiazol-2-yl-oxy)-acetic acid 2-ethylpiperidine and (5-trifluoromethyl-1,3,4-thiadiazol-2-yl-oxy)-acetic acid 2-methylpiperidine. In accordance with those synthetic schemes, the ethylpiperidine compound is made by reacting hydroxyacetic acid-2-ethylpiperidine with 2-chloro-5-trifluoromethyl-1,3,4-thiadiazole in the presence of potassium tert-butanate in tert-butanol at a temperature of 20.degree. C. to 30.degree. C. for 3 hours and the methylpiperidine compound is made by reacting 2-chloro-5-trifluoromethyl-1,3,4-thiadiazole in toluene with hydroxyacetic acid-2-methylpiperidine in the presence of sodium hydroxide. The end product, in both cases, is recovered from the reaction mixture by acidification with hydrochloric acid, drying, removal of solvent and crystallization. The reported yields of the end-products were 66% (ethylpiperidine) and 54% (methylpiperidine).
U.S. Pat. Nos. 4,968,342 and 5,090,991 disclose a synthetic scheme for making N-isopropyl-(5-trifluoromethyl-1,3,4,-thiadiazol-2-yl)-3'-chlorooxyacetani lide. In accordance with that scheme, 2-methylsulfonyl-5-trifluoromethyl-1,3,4-thiadiazole, dissolved in acetone, with 3'-chloro-N-isopropylhydroxyacetanilide in the presence of sodium hydroxide and water for 3 hours at -20.degree. C. Water is added to the reaction mixture and the crystalline end-product obtained by crystallization in 85% yield.
It can be seen from the above, that existing methods for making acetamide herbicides suffer from low yields (54% to 85%), prolonged reaction times (20 to 40 hours) or the use of problematic solvents (acetone). There continues to be a need in the art, therefore, for a practical method for making these herbicides, which method avoids the problems of the existing art.